Recently, a hybrid vehicle and an electric vehicle are widely attracting attention as environment-friendly vehicles. The hybrid vehicle has partly been brought into practical use.
The hybrid vehicle is a vehicle powered by a DC (Direct Current) power source, an inverter, and a motor driven by the inverter, in addition to a conventional engine. Specifically, while it is powered by driving the engine, it is also powered by converting DC voltage from the DC power source by the inverter to AC (Alternating Current) voltage and rotating the motor by thus converted AC voltage. Additionally, the electric vehicle is a vehicle powered by a DC power source, an inverter and a motor driven by the inverter.
In such a hybrid vehicle or an electric vehicle, it has also been contemplated to increase DC voltage from a DC power source by a DC/DC converter and to supply the increased DC voltage to an inverter driving a motor (see Japanese Patent Laying-Open No. 8-214592).
Specifically, the hybrid vehicle or the electric vehicle is equipped with a motor driving apparatus shown in FIG. 19. Referring to FIG. 19, a motor driving apparatus 300 includes a DC power source B, system relays SR1 and SR2, capacitors 308 and 322, a bidirectional converter 310, a voltage sensor 320, and an inverter 330.
DC power source B outputs DC voltage. System relays SR1 and SR2, when being turned on by a control apparatus (not shown), supply the DC voltage from DC power source B to capacitor 308. Capacitor 308 smoothes the DC voltage supplied from DC power source B via system relays SR1 and SR2, and supplies the smoothed DC voltage to bidirectional converter 310.
Bidirectional converter 310 includes a reactor 311, NPN transistors 312 and 313, and diodes 314 and 315. Reactor 311 has its one end connected to a power supply line of DC power source B, and has its other end connected to an intermediate point between NPN transistor 312 and NPN transistor 313, i.e., between an emitter of NPN transistor 312 and a collector of NPN transistor 313. NPN transistors 312 and 313 are serially connected between the power supply line and an earth line. NPN transistor 312 has its collector connected to the power supply line, while NPN transistor 313 has its emitter connected to the earth line. Between the collectors and the emitters of NPN transistors 312, 313, diodes 314, 315 for passing current from the emitter side to the collector side are connected, respectively.
Bidirectional converter 310 has its NPN transistors 312 and 313 turned on/off by the control apparatus (not shown), and increases the DC voltage supplied from capacitor 308 to supply the output voltage to capacitor 322. Further, at a regenerative braking of the hybrid vehicle or the electric vehicle equipped with motor driving apparatus 300, the bidirectional converter 310 reduces the DC voltage generated by AC motor M1 and converted by inverter 330 and supplies it to capacitor 308.
Capacitor 322 smoothes the DC voltage supplied from bidirectional converter 310 and supplies the smoothed DC voltage to inverter 330. Voltage sensor 320 detects output voltage Vm of capacitor 322.
When supplied with the DC voltage from capacitor 332, inverter 330 converts the DC voltage to AC voltage based on the control from control apparatus (not shown) to drive AC motor M1. Thus, AC motor M1 is driven to produce the torque specified by a torque instruction value. Further, inverter 330 converts the AC voltage generated by AC motor M1 based on the control from the control apparatus to DC voltage, and supplies thus converted DC voltage to bidirectional converter 310 via capacitor 322.
Further, the hybrid vehicle is equipped with a motor driving apparatus 400 shown in FIG. 20. Referring to FIG. 20, motor driving apparatus 400 includes an inverter 340 in addition to the configuration of motor driving apparatus 300. The rest of the configuration is identical to that of motor driving apparatus 300.
Inverter 340 converts the DC voltage from capacitor 322 to the AC voltage based on the control from a control apparatus (not shown) to drive an AC motor M2 by thus converted AC voltage. Thus, AC motor M2 is driven to produce the torque specified by a torque instruction value. Further, inverter 340 converts the AC voltage generated by AC motor M2 to DC voltage based on the control from the control apparatus, and supplies thus converted DC voltage to bidirectional converter 310 via capacitor 322.
However, if power P transferred to AC motor M1 abruptly increases at time point t0 in motor driving apparatuses 300 and 400 as shown in FIGS. 21A and 21B, a storage voltage Vm of capacitor 322 decreases accordingly.
When a response time constant of DC/DC converter 310 at this time is tf, Vm<Vb (battery voltage) is invited if power P is abruptly transferred during tf. As a result, a problem may arise that current flows from DC power source B side to the output side of DC/DC converter 310 via diode 314 of DC/DC converter 310 without limitation, and DC/DC converter 310 can not increase the DC voltage from DC power source B with any selection of duty ratio.
Specifically, DC/DC converter 310 can not address the transfer of energy from capacitor 322 when the power from AC motor M1 abruptly changes due to the effect of reactor 311 accommodated in DC/DC converter 310, and therefore output voltage Vm of capacitor 322 decreases. Thus, an overcurrent occurs from DC power source B for recovering the dropped output voltage Vm of capacitor 322. The element of a chopper may be damaged if such a situation is continued.
Further, a rush current increases due to the drop of the output voltage of capacitor 322. Therefore, the damage of DC power source B worsens by the large current being transferred from DC power source B.
Still further, motor driving apparatus 400 has been involved with the following problem: When AC motor M1 is consuming the electricity stored in capacitor 322 and AC motor M2 is generating electricity, if AC motor M1 stops to consume the electricity stored in capacitor 322, then the DC power returned from inverter 340 to capacitor 322 abruptly increases, and the voltage being applied to bidirectional converter 310 abruptly increases. Then, bidirectional converter 310 will not be capable of following the abrupt increase of the voltage and therefore an overvoltage is applied thereto. As a result, there has been a problem that motor driving apparatus 400 may not operate normally.